The present invention relates to an improved mixing device and a mixing method, and coding and decoding devices and methods employing such an improved mixing device or method.
The present invention also relates to a improved mixing device for mixing audio signals which minimizes signal deterioration resulting from multi-stage connection of mixing circuits, as well as an audio system using such an improved mixing device.
As known in the art, the CODEC (abbreviation of Coder-Decoder) is an indispensable means to enable information communication between an analog signal system and a digital signal system, and generally comprises an A/D converter for converting an analog signal into a digital signal and a D/A converter for converting a digital signal into an analog signal. In recent years, CODECs employing a so-called xe2x80x9coversamplingxe2x80x9d technique have been attracting the attention of many people who desire to simplify the analog circuitry contained therein. In these CODECs, analog signals are converted, via an A/D converter based on a delta sigma modulation scheme, into bitstream data of a much higher bit rate than the frequency of the analog signals. The bitstream data are then subjected to a filtering process, xe2x80x9cdecimationxe2x80x9d process, etc. to provide high-quality PCM data of minimized noises.
Also, with today""s technological diversification, there is an increasing demand that the CODECs should have various other functions than the above-noted signal format conversion, among which is a mixing function to synthesize a plurality of input signals. To meet such a demand with the CODEC using the oversampling technique, it may be useful to first mix or synthesize a plurality of analog signals and then convert them into bitstream data, or to first convert a plurality of analog signals into bitstream data and then subject the data to a xe2x80x9cdecimationxe2x80x9d process (i.e., process to convert the data into low-sampling-frequency and multiple-bit data), followed by mixing of these data.
However, the above-noted mixing technique has various problems as follows.
(1) Problem of Electrical Characteristics:
When the required mixing is performed on analog signals, unwanted entry of noises into the signals and distortion of the signals would unavoidably result during the mixing. Such noises and signal distortion may be reduced by use of a good-performance analog circuit, which, however, makes it necessary to apply relatively high electric current to the analog circuit, thus leading to in increased overall electric current consumption by the CODEC.
(2) Problem of Costs:
The mixing analog circuit has to have high resistance to adjust the mixing level, which would require a great space to accommodate a level adjusting resistor in an IC circuit implementing the CODEC, thereby increasing the size or area of the IC circuit chip. In addition, it is generally difficult to reduce the size of analog circuit. Therefore, in cases where the mixing circuit is analog one, reduction in size of the CODEC IC circuit is hard to achieve.
(3) Problem of Productivity:
Normally, after the CODEC IC circuit is manufactured, a test is carried out to check the performance of the IC circuit, in which it is necessary to determine whether or not the analog circuit properly performs a desired mixing function. However, this determination would be time-consuming because it has to be done via analog measurement using an analog tester.
(4) Problem of Quality:
The analog circuit is more susceptible to manufacturing unevenness or variations than the digital counterpart. Thus, in cases where the mixing circuit is analog, it is more likely that mixing circuits unable to provide desired performance will be produced.
The problems as set forth above are found not only in CODECs but also various other devices that require a function to mix bit stream data.
There are known two audio-signal mixing methods: one of the two is to synthesize a plurality of audio signals in analog form; and the other is to synthesize a plurality of audio signals in the form of digitized PCM data. The first-said method allows the necessary mixers to be constructed at a relatively low cost but is susceptible to noises, so that intolerable signal deterioration would often occur at the last-stage mixing output if the mixing circuits are connected in a multi-stage fashion. Further, the second-said method can avoid the problems of unwanted noises and signal deterioration because digitized PCM data are synthesized in this method; however, the overall cost of a device employed would substantially increase because channel-by-channel synchronization is required.
It is therefore a primary object of the present invention to provide a mixing device and a mixing method which are capable of mixing bitstream data without having to convert the data into analog representation, as well as coding and decoding devices and methods employing such a mixing device or method.
It is a second object of the present invention to provide an audio signal interface device which is simple in construction and yet permits appropriate input of analog audio signals of plural channels.
It is a third object of the present invention to provide a mixing device which can perform high-quality mixing at low cost with minimized influence of noises and signal deterioration, as well as an audio system employing such a mixing device.
In order to accomplish the above-mentioned objects, the present invention provides a mixing device for mixing plural bitstream data, which comprises: a weighting circuit that weights a plurality of bitstream data for conversion thereof into respective plural-bit digital data; an adder circuit that adds together the respective plural-bit digital data outputted from the weighting circuit, so as to output added digital data; and a converter circuit that converts the added digital data into bitstream data.
The xe2x80x9cbitstream dataxe2x80x9d is pulse-density-modulated (PDM) data and has a logical value xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d at each of time slots corresponding to a predetermined bit rate. A plurality of bitstream data to be mixed are weighted for conversion into respective plural-bit digital data. That is, by being weighted with plural-bit digital coefficients of optionally selected values, the bitstream data to be mixed are converted into respective plural-bit digital data, i.e., data in PCM representation (pulse-code-modulated data). The weighting digital coefficients perform not only a function of converting the bitstream data into PCM data but also a function of setting a desired mixing ratio. Thus, the weighting degrees for the individual bitstream data may be variably set as desired. Then, the converted PCM data are added together, and the resultant added PCM data is reconverted into bitstream data (PDM data). In the above-mentioned manner, the plurality of bitstream data are mixed together through full-digital processing without having to use analog circuitry, and the mixed result is provided as one bitstream data. The converter circuit for converting the PCM data into the bitstream data (PDM data) may comprise a xcex94xcexa3 (delta sigma) modulator. The delta sigma modulator includes an integrator circuit in its input section, which is advantageously capable of removing shaping noise components from the original bitstream data (PDM data).
The mixing device according to the present invention is applicable not only to mixing between bitstream data but also to mixing between bitstream data and PCM data.
Namely, the mixing device of the present invention may comprises: a weighting circuit that receives bitstream data of a predetermined bit rate and weights the bitstream data for conversion thereof into plural-bit digital data; a PCM data supply circuit that supplies PCM digital data of a sampling rate corresponding to the bit rate; an adder circuit that adds together the digital data outputted from the weighting circuit and the digital data supplied from the PCM data supply circuit, so as to output added digital data; and a converter circuit that converts the added digital data, outputted from the adder circuit, into bitstream data, whereby the bit stream data outputted from the converter circuit is a mixture of the bitstream data and the PCM digital data.
By inclusion of the above-mentioned mixing device, it is possible to provide a coding device which is capable of mixing a plurality of analog signals and coding the mixed result without having to use analog circuitry. Namely, the coding device of the present invention comprises: a first converter circuit that converts the analog signals into respective bitstream data; a weighting circuit that weights the respective bitstream data for conversion thereof into respective plural-bit digital data; an adder circuit that adds together the respective plural-bit digital data outputted from the weighting circuit, so as to output added digital data; a second converter circuit that converts the added digital data, outputted from the adder circuit, into bitstream data; and a PCM conversion circuit that converts the bitstream data, outputted from the second converter circuit, into PCM data of a predetermined sampling rate.
Further, the present invention provides a coder-decoder (CODEC) device for receiving an analog signal and a digital PCM signal and outputting a mixture of the received analog signal and digital PCM signal, which comprises: a first converter circuit that converts the analog signal into bitstream data of a predetermined bit rate; a weighting circuit that weights the bitstream data, outputted from the first converter circuit, for conversion thereof into plural-bit digital data; an interpolator circuit that oversamples the received PCM signal until a sampling rate corresponding to the bit rate is reached and then interpolates between digital PCM data resultant from oversampling of the PCM signal; an adder circuit that adds together the plural-bit digital data outputted from the weighting circuit and digital PCM data outputted from the interpolator circuit, so as to output added digital data; a second converter circuit that converts the added digital data, outputted from the adder circuit, into bitstream data; and a PCM conversion circuit that converts the bitstream data, outputted from the second converter circuit, into PCM data of a predetermined sampling rate.
With such an arrangement, a mixture of the received analog signal and digital PCM signal is provided, as data in PCM representation, by the PCM conversion circuit. There may also be provided an analog low-pass filter that receives the bitstream data from the second converter circuit and provide analog data indicative of a mixture of the received analog data and digital PCM signal.
The present invention also provides an audio signal interface device which comprises: a supply circuit that supplies predetermined clock signals; a converter circuit that converts analog audio signals of plural channels into respective one-bit bitstream data at a bit rate based on the clock signals; and a multiplexer circuit that time-divisionally multiplexes the respective one-bit bitstream data of the individual channels on the basis of the clock signals in such a manner that the analog audio signals of the plural channels are converted into one-bit multiplexed bitstream data. The converter circuit may comprise a delta sigma modulator.
In such an arrangement, analog audio signals of plural channels, such as analog audio signals of left and right channels in a stereophonic system, are input and converted into respective one-bit bitstream data. Then, the respective one-bit bitstream data of the individual channels are time-divisionally multiplexed to provide one-bit multiplexed bitstream data. Thus, no particular processes for synchronizing, packeting, decoding purposes are required, which can greatly simplify the construction of an interface for receiving analog audio signal of plural channels. The one-bit multiplexed bitstream data is demultiplexed in a device (e.g., personal computer) utilizing the interface device and then subjected to a decimation process to be coded into plural-bit PCM data.
The present invention also provides an audio signal mixing device which comprises: a supply circuit that supplies predetermined clock signals; at least two audio signal interfaces, each of said audio signal interfaces including a first converter circuit that converts audio signals of plural channels into respective one-bit bitstream data at a bit rate based on the clock signals, and a multiplexer circuit that time-divisionally multiplexes the respective one-bit bitstream data of the plural channels on the basis of the clock signals in such a manner that the audio signals of the plural channels are converted into one-bit multiplexed bitstream data; a weighting circuit that weights the multiplexed bitstream data, outputted from each of the audio signal interfaces, for conversion into plural-bit digital data; an adder circuit that adds together the plural-bit digital data outputted from the weighting circuit, so as to output added digital data; and a second converter circuit that converts the added digital data, outputted from the adder circuit, into bitstream data, so as to output single time-divisionally multiplexed bitstream data resultant from addition by the adder circuit.
With such an arrangement, audio signals of plural channels, such as audio signals of left and right channels in a stereophonic system, are converted into one-bit multiplexed bitstream data in each of the audio signal interfaces. The multiplexed bitstream data from the audio signal interfaces are then digitally weighted and the resultant weighted data are added together on a channel-by-channel basis to provide single time-divisionally multiplexed bitstream data. Thus, the circuit structure can be extremely simplified. Besides, because all necessary processes are automatically synchronized on the basis of a common clock signal, no particular synchronizing circuit is required, which thus simplifies the overall circuitry structure. Further, because the signals and data the invention are all processed in full digital form, they are quite insusceptible to noise and practically no signal deterioration occurs even where a plurality of mixing circuit are connected in a multi-stage fashion.